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Milling Cutters.

177

narrower on re-grinding. The space between the teeth is to
admit an emery wheel for grinding the faces.

Angle of tooth, although important, is still rather in dispute,
principally because the same cutter, to avoid expense, is being
used for various materials—a wrong procedure, without doubt,
Probably some variation on the angles already given is necessary,
because of the higher speed of cut. Experience seems to suggest
the following:—

Cutting angle ..................    80° to tangent.

Angle of relief..................    10° to tangent.

Front rake  .....................    10° to radius.

giving a tool angle of 70°. Small mills are made with radial
teeth, corresponding to a cutting angle of 90°. A side rake of
10° should be given, and the teeth cut spirally or obliquely on a
finishing tool.

Speeds. —-There is still more variation in practice regarding
these. They can be considerably higher than for other tools,
because each tooth is in contact for only a small portion of the
revolution, and has ample time to cool. The result is the more
highly finished work that has brought milling into favour. The
following speeds give the result of experience, and are fairly
correct:—

Milling Speeds in feet per minute ; and revolutions per minute, in
terms of radius (r)" of cutter.

For Steel      . ...........
	ROUGH i Ft. per M.
	NG CUT. Rev. perM.
	FlNISHI
 Ft. per M.
	SG CUT. Rev. per M.

	30 40 60 80
 IOO
	51. r
 9i
 r
 11± r
 JL5£ r 190 r
	40
 55
 .75
 IOO I2O
	i.6..
 r
 !°1 r
 *i3 r 190 r 228 r

„   Wrought Iron  ......
				
„   Cast Iron ...........
				
„   Gun Metal    .........
				
,,   Brass...,. .............